Quality Confirmation Tests for Power Transformer Insulation Systems (eBook)

Preface 5
Contents 6
1 Unused Mineral Insulating Oil 10
1.1 Introduction 10
1.2 Mineral Oil 10
1.3 Classification of Mineral Oil Based on Application 12
1.3.1 Transformers Oil 12
1.3.2 Switchgear Oil in Low Temperatures 12
1.4 Additives 17
1.5 Special Cases 18
1.6 Analysis of Potentially Corrosive Sulphur 18
1.7 Oil Contamination 19
References 20
2 In-Service Mineral Insulating Oil 21
2.1 Introduction 21
2.2 Oil Monitoring and Purification 21
2.3 Oil Ageing and Degradation 22
2.4 Oil Tests 22
2.4.1 Color and Appearance 23
2.4.2 Breakdown Voltage 23
2.4.3 Water Content 23
2.4.4 Water in the Insulation System 25
2.4.5 Acidity 27
2.4.6 Dielectric Dissipation Factor (DDF) and Resistivity 28
2.4.7 Additives and Oxidation Stability 29
2.4.8 Sludge and Sediment 30
2.4.9 Interfacial Tension 30
2.4.10 Particle Content 31
2.4.11 FlashPoint 31
2.4.12 Compatibility of Insulating Oil 31
2.4.13 Pour Point 32
2.4.14 Density 33
2.4.15 Viscosity 33
2.4.16 PCB 34
2.4.17 Corrosive Sulphur 34
2.4.18 Dibenzyl Disulphides (DBDS) 35
2.4.19 Passivators 35
2.5 In-Service Oil Monitoring 35
2.5.1 Uninhibited Oil Monitoring 36
2.5.2 Inhibited Oil Monitoring 36
2.6 Time Schedule of Sampling and Testing In-Service Oil 36
2.7 Available On-Site Tests 37
2.8 Classification of Operating Oil 37
2.9 Corrective Actions 38
2.10 Purification 38
2.10.1 Physical Purification 39
2.10.2 Chemical Purification (Refinement) 40
2.11 Replacing Oil in Electrical Equipment 42
2.12 Adding Passivators 43
2.13 Determining Water Concentration in the Oil 43
References 44
3 Chemical Indicators 45
3.1 Introduction 45
3.2 Insulation Paper Life Determination 45
3.3 Cellulose 47
3.4 Cellulose Molecular Structure 47
3.5 Cellulosic Insulation 48
3.6 Degree of Polymerization 48
3.7 Oil Impregnated Insulation Paper 49
3.8 Ageing of Oil Impregnated Insulation Paper 50
3.9 Ageing Mechanism 51
3.9.1 Pyrolysis 51
3.9.2 Hydrolysis 52
3.9.3 Oxidation 54
3.10 Influence from Acids 54
3.11 Ageing of Oil 55
3.12 Oil Oxidation 55
3.13 Degradation Products in Oil Impregnated Insulation Systems 55
3.14 Degradation Products from Cellulosic Insulation 56
3.14.1 Water 56
3.14.2 Acids 56
3.14.3 Furans 57
3.14.4 Carbon Oxides 57
3.14.5 Hydrocarbons 58
3.15 Degradation Products of Oil 58
3.15.1 Acids 58
3.15.2 Sludge 58
3.16 Chemical Indicators 58
3.17 Furan Compounds 59
3.17.1 Furans Origin 59
3.18 The Relationship Between DP and Furans 60
3.19 Stability 61
3.20 Furans Disadvantages 61
3.21 CO2 and CO 62
3.22 The Combination of CO2/CO Ratio and 2-Furfural 62
3.23 Methanol 63
References 70
4 Dissolved Gas Analysis (DGA) 73
4.1 Introduction 73
4.2 Total Flammable Dissolved Gas in the Transformer 75
4.3 Allowable Concentration of Gases in a Transformer 75
4.4 Gas Ratio Methods 75
4.4.1 Dürrenberg Method 76
4.4.2 Rogers Ratio 77
4.4.3 IEC Ratio Method 77
4.5 Duval Triangle Method 79
4.6 Detection of Partial Discharge Using DGA 80
4.7 Impact of DGA Accuracy on Fault Detection 80
References 81
5 Other Tests 82
5.1 Introduction 82
5.2 Partial Discharge (PD) 83
5.2.1 Corona Discharge 83
5.2.2 Surface Discharge 83
5.2.3 Discharge in Composite Insulation Materials 85
5.2.4 Electric Discharge in Cavities 85
5.2.5 Electric Treeing 85
5.3 Partial Discharge Measurement 85
5.4 Insulation Monitoring by PD Measurement 88
5.5 Comparison of Electrical and Audio Detection Methods 90
5.6 Partial Discharge Formation in Transformers 91
5.7 Dielectric Response Analysis 91
5.8 Polarization 92
5.9 Polarization and Depolarization Current 92
5.10 Insulation Spectroscopy in Time Domain 95
5.11 FDS Test 98
5.12 Returning Voltage Method 100
5.13 Isothermal Relaxation Current 101
5.14 Frequency Response Analysis (FRA) 103
5.15 Frequency Response Analysis Theory 104
5.16 Application of FRA in Power Transformers 104
5.17 FRA Test Features 105
5.18 Frequency Response Measurement Methods 105
5.18.1 Swept Frequency Method (SFM) 105
5.18.2 Low Voltage Impulse Methods (LVI) 106
5.19 Comparison of LVI and SFM Methods 106
5.20 Detectable Defects by FRA 108
References 109
Index 112